The present invention relates to sealed actuators, and more particularly to a sealed actuator adapted for use in an ultra-high vacuum environment in which even small amounts of contaminants and impure gases are not admitted or in an environment in which magnetic poles and coils of a motor become corroded, such as in a corrosive gas environment.
For example, in semiconductor manufacturing, a workpiece is worked in an ultra-high vacuum environment in order to eliminate impurities to a possible extent. In an actuator employed in such a case, a lubricant that contains volatile component such as ordinary grease is not allowed to be used for bearings of a drive shaft of a drive motor for, e.g., a workpiece positioning apparatus. Therefore, the inner and outer races of such bearings are plated with soft metal such as gold or silver. Further, the coil insulators and wiring sheathes of the motor, the adhesives of laminated magnetic poles, and the like are made of stable materials having good heat resistance and discharging relatively small amounts of gases.
On the other hand, as means for introducing rotational output into an ultra-high vacuum vessel from outside, various types of actuators such as a bellows type drive system, a magnetic coupling drive system, a magnetic fluid seal drive system have heretofore been known. These actuators are so constructed that the output end of a rotating shaft supported by vacuum bearings is projected into the vacuum environment and that rotating force is transmitted to the input end by a drive apparatus disposed in the atmosphere. More specifically, a bellows type drive system is designed as follows. As shown in FIG. 6, an output end 101A of a rotating shaft 101 is projected into a vacuum environment V while being supported by vacuum bearings 102, and when an inclined-plate type oscillating mechanism 103 on the other end 101B is driven to rotate by a rotating apparatus 105 disposed in the atmosphere, a bellows 104 repeats expansion and contraction, so that the rotating shaft 101 is rotated.
In contrast thereto, a magnetic coupling drive system is designed as follows. A rotor made of a magnetic substance is secured to the input end of a rotating shaft with the outer circumference of the rotor being hermetically sealed by a housing. A magnet is arranged on the side of the atmosphere so as to surround the rotor while the housing being interposed. The magnet is rotated to thereby rotate the rotating shaft.
Further, a magnetic fluid seal drive system is designed as follows. A housing made of a nonmagnetic substance is arranged so as to pass through a partition wall disposed between the atmosphere side and the vacuum side. Not only circular-ring-like pole pieces sandwiching a permanent magnet therebetween are arranged between bearings disposed in the housing, but also a clearance between the outer circumferential surface of a rotating shaft passing through the housing and the inner circumferential surface of the pole pieces confronting the outer circumferential surface of the rotating shaft is sealed with a magnetic fluid.
Recently, higher integration of semiconductors is accompanied with higher density by miniaturizing IC pattern widths. In order to fabricate wafers that can meet such miniaturization needs, a high degree of consistency for wafer quality is required. To meet such needs, it is important to reduce impure gas concentration in wafers in a low-pressure gas processing chamber. Further, in order to implement miniaturization as required, an extremely highly accurate positioning apparatus must be employed.
From these viewpoints, the aforementioned conventional actuators have the following problems.
In the case of a drive motor used for an ultra-high vacuum apparatus:
(1) Even if highly heat-resistant, stable materials that discharges relatively small amounts of gases are selected for the coils, insulators, wiring sheathes, and the like of the drive motor, these materials still impose problems as long as they are organic insulating materials. Since an organic insulating material is porous and has numerous holes over the surface when observed microscopically. When such material is exposed to the atmosphere, gas, moisture, and the like are adsorbed into the holes on the surface thereof. It takes much time to degas such adsorbed impure molecules by means of evacuation, which is most likely to reduce production efficiency.
(2) In addition, no heat radiation by air convection occurs in a vacuum. Therefore, if coil temperature increases locally, resistance at such local part increases to accelerate heating, which in turn makes the coil insulating film susceptible to burning.
(3) On the other hand, it is conceivable to reduce adsorbed impure molecules by using inorganic materials for coil insulators and sheathed wires in stainless conduits for wiring. However, this measure not only entails large costs, but also imposes the problem that the motor capacity is reduced due to the fact that the rate of conductors such as copper in the coil winding space is reduced to increase electric resistance.
In contrast to the aforementioned problems imposed when an actuator is disposed in an ultra-high vacuum apparatus, the following problems arise in the cases where the drive section of an actuator is disposed outside a vacuum apparatus as in the bellows type drive system, the magnetic coupling drive system, the magnetic fluid seal drive system, and the like.
In the bellows type drive system, large backlash occurs. In the magnetic coupling drive system in which rotating force is transmitted by the attracting force of a magnet, rigidity is reduced. That is, highly accurate positioning requirements cannot be met by these systems.
Further, in the magnetic fluid seal drive system, the magnetic fluid has a heat resistance temperature of about 70° C., which is a relatively low temperature. Therefore, the magnetic fluid is not resistant to beating temperatures during a bake-out process in an ultra-high vacuum vessel (the process of discharging adsorbed gas and water molecules contained in an inner wall of a vacuum vessel and the like). As a result, the magnetic fluid, containing a small amount of volatile component, discharges gas disadvantageously.
To overcome these problems of the conventional actuators, the present applicant proposed a sealed actuator in Japanese Patent Unexamined Publication Nos. Hei. 3-150041 and Hei. 3-150042. This actuator is characterized by discharging no impure gas in an ultra-high vacuum environment and achieving highly accurate positioning. This actuator includes: a motor stator having rotation-drive magnetic poles excited by rotation-drive coils; a motor rotor arranged so as to confront the magnetic pole surfaces of the motor stator while having a small clearance with respect to the magnetic pole surfaces and rotatably supported through roller bearings; and a resolver serving as a displacement detecting means for measuring a displacement of the motor rotor. The actuator has a partition wall made of a nonmagnetic metal between the motor stator and the motor rotor so that the inner space within which the motor stator is disposed is hermetically covered with the partition wall, which in turn allows the motor stator side space to be isolated from the motor rotor side space.
In the sealed actuator described above, since the motor stator is isolated from the motor rotor by the partition wall made of a nonmagnetic metal, even if the. actuator is used in a high vacuum environment or reactive gas environment of a semiconductor manufacturing apparatus, neither impure gases are discharged from the coils and organic insulators of the actuator to contaminate the environment nor are the coils and organic insulators eroded. In addition, the formation of a magnetic circuit is not hindered between the motor stator and the motor rotor. Moreover, highly accurate positioning can be implemented by the resolver. Thus, such actuator is highly useful in practical use.
However, the thickness of the partition wall made of a nonmagnetic metal must be so limited as not to hinder the formation of a magnetic circuit between the motor stator and the motor rotor in particular. Thus, when exposed to an ultra-high vacuum, the partition wall may be swollen.
Further, as a drive apparatus of a magnetic coupling drive system, the configuration as shown in FIG. 7 is known. That is, an attachment flange 201 is attached to an opening of a bottom wall 202 of a vacuum container. In the inside of housings 216 and 236 positioned outside of the vacuum container, two drive shafts of an outer drive shaft 204 and an inner drive shaft 205 are coaxially disposed and extend outside of the housing through the opening. The outer drive shaft 204 positioned in the vacuum container is supported by bearing 206 at the tip portion of the inner drive shaft 205.
Further, a motor rotor 207 is supported on an outer surface of the outer drive shaft 204. A motor stator 208 corresponding thereto is supported on an outer housing 216 of the motor rotor 207. Similarly, a motor rotor 209 is supported on an outer surface of the inner drive shaft 205. A motor stator 210 corresponding thereto is supported on an outer housing 236 of the motor rotor 209. The motor rotors 207 and 209 are disposed in a vacuum state, and the motor stators 208 and 210 are disposed outside of the vacuum state.
The outer drive shaft 204 is supported on the housing 216 through bearings 218 and 219, and the inner drive shaft 205 is supported on the housing 236 through bearings 238 and 239. Between the motor rotor 207 and the motor stator 208, and between the motor rotor 209 and the motor stator 210, thin nonmetal partition walls 216a and 236a extended from the housing 216 and the housing 236 are respectively located to keep the vacuum state in the side of the motor rotor 207 and 209.
In such a configuration, for the improvement of performance of a motor, it is required to prevent the decrease in magnetic flux to a possible degree between the motor rotor and the motor stator by the nonmagnetic partition wall. For the purpose, the thickness of the partition wall must be as thin as possible. Thus, since the outer drive shaft 204 and the inner drive shaft 205 are supported by the bearings disposed in the housings 216 and 236 including the thin partition wall, the conventional drive apparatus has a problem that supporting rigidity of the respective drive shafts to the housings is lowered. If an arm or the like is attached to the tip of the drive shaft of the drive apparatus having such a structure and a load is applied to the tip, the force acting on the bearings acts also on the partition wall so that such a possibility can not be neglected that the partition wall is deformed or the partition wall is broken, which is a problem of the conventional apparatus.
Further, since the support rigidity of the outer drive shaft 204 and the inner drive shaft 205 are low, there occurs a problem that both the drive shafts are brought into contact with each other by swing due to rotation of both the drive shafts. Accordingly, this prior art overcomes the disadvantage of contact of both the drive shafts by using the pilot bearings 206.
Moreover, high integration of semiconductors requires control of higher accuracy and stability. Under such circumstances, positioning control with a resolver becomes insufficient due to the fact that magnetism from a motor stack surrounds the resolver.